With the advent of various state and federal regulations pertaining to the inspection of automobiles and other vehicles for excessive exhaust pollution and the like, it is desirable if not necessary to have devices which can properly evaluate a vehicle's performance during operation and provide an indication as to whether or not the vehicle is operating properly from an air pollution standpoint. In some cases, this requires that the vehicle's engine run at a predetermined speed (RPM) during tests such as exhaust gas evaluation. Thus, in the case of state inspection agencies responsible for testing these vehicles, when carbon monoxide (CO) and hydrocarbons (HC) are measured during idle conditions (typically 500-800 RPM) or at higher engine speeds of approximately 2500 RPM, it is necessary to know within a permissible deviation the RPM of the engine of the vehicle undergoing testing. Obviously, there are other occasions (e.g., regular performance or pre-inspection checkups) when the vehicle owner will desire to have an automotive exhaust evaluation made. Preferably, and particularly in high volume inspection situations, it is desirable to have a device that can provide a measure of the engine speed without the need for a physical connection between the device and the engine of the vehicle being tested, as a companion inspection tool for the easily inserted tail pipe probe used for HC and CO concentration measuring instruments.
It is also necessary to provide an RPM measurement of engine speed which is relatively accurate and preferably in the range of about 5% of the actual engine speed. Typically, conventional methods for detecting signals generated by the spark ignition system of a vehicle's engine have been able to provide good results only by very close coupling of the pickup device to the high tension leads from the spark coil or by direct connection to the spark coil primary circuit on the spark-ignited engine. In these systems, the electrical signal for each firing is detected by the pickup device and processed through suitable electronic equipment, whereupon engine RPM is indicated on a suitable readout means. A typical prior art disclosure of such a system is disclosed in U.S. Pat. No. 3,601,773.
Another requirement for a preferred system is that it be selective to receive only ignition signals, and substantially free of interference from nearby operating vehicle engines which may be undergoing test or just in the vicinity for some other reason. Other unsuccessful prior art attempts at overcoming these and other related problems also include U.S. Pat. Nos. 3,573,609 and 3,656,059; and further prior art considered in connection with this invention include U.S. Pat. Nos. 3,611,138, 3,005,155 and 3,404,339.
The prior art, while generally disclosing the broad concept of providing a remote tachometer for sensing engine speed, without the need for a physical connection between the engine and the tachometer, the various problems associated with the prior art (e.g., interference by surrounding operating engines) have made such prior art devices and systems impractical. In connection with the production by spark-ignition engines of radio signals with each ignition pulse, and the processing of such signals to provide an indication of engine speed, a significant problem associated therewith is that these signals (radio frequency) emitted by the ignition system vary in intensity from cylinder to cylinder for each particular engine and car, and also from car to car. The RF signals from cylinder to cylinder vary due to mixture conditions at the gap when the plug fires and due to variations in plug and wiring conditions. As for the RF signals between cars, they vary because of ignition system design and associated conditions. Further, various other devices in the car will generate RF signals which cause false pulses to appear at the pickup device. Finally, a highly sensitive pickup device will pick up RF signals not associated with the car and which will cause further false signals to appear. By the use of the term "RF signal", it is meant to include such signals that fall within a frequency range from approximately 10 KC and below to 100 MC.